It is well known that any body whose temperature is greater than absolute zero emits radiation in the infrared range the spectral intensity of which depends on the nature of the emitting surface and the absolute temperature of the body. In many military, medical and industrial uses it is desirable to convert infrared radiation into visible light in order to observe bodies and measure their surface temperatures.
There are systems in which the infrared radiation is converted into visible light by means of image amplifiers. However, these systems only use a small part of the infrared spectrum, i.e. near visible light, in the range between 0.8 .mu.m and 1.4 .mu.m. As for bodies at ambient temperature the greater part of their emitted infrared energy is lost since it is located near 10 .mu.m and cannot be utilized in this manner.
Other systems employ the near and remote infrared spectrum between 1 .mu.m and 100 .mu.m, for example, motion-picture camera with a thermocouple, holometer or pyroelectric type thermal detecters. These instruments require about one minute to provide an image. They are therefore incapable of monitoring moving bodies.
There are instruments which monitor moving bodies at a speed close to the standards of conventional television. To achieve such results quantic receivers are employed which must be cooled with liquid helium or liquid oxygen which is a considerable constraint.
In another system which requires about 5-6 seconds to produce a visible image, a liquid is used which is locally condensed or evaporated in greater or lesser amounts depending on the intensity of incident infrared radiation. However, apart from the fact that it needs several seconds to produce an image, which is still too long to observe moving bodies, the system must be reset to observe another image which is at least a one-minute operation even in the hands of an experienced technician.
To enable the monitoring of moving bodies or events in the infrared range, there are some devices which employ a sensitive layer which vary locally in accordance with the radiation intensity. Such local variations are usually deformations of the surface of the sensitive layer. The resulting relief image is used to control a visible light which after being reflected or transmitted by the deformed surface passes through a viewing system, e.g. a Schlieren, phase contrast or holographic system.
The difficulty with such systems lies in the choice of a suitable sensitive layer. Such layers are often of low sensitivity, and/or difficult technically to produce. For example, the part of the fine solid membrane heated by the infrared radiation expands; the neighbouring regions which are not heated or differently heated oppose the expansion of the contour of the first part thereby producing a two-dimensional warping.
It has also been proposed (see French patent No. 1,452,665) an image converter in which the control layer is a thin film of a low viscosity liquid, e.g. a hydrocarbon. The sensitive layer then comprises a thin liquid film which absorbs radiation and heats up locally. There results a variation in density and convection motion is established which may be observed if the liquid contains fine particles in suspension. Under the same conditions the free surface of the liquid deforms and produces an infrared relief image located in the liquid. Finally, a sensitive layer may be formed as a semi-conductor. In this case it is not the surface which deforms but the refractive index which changes in accordance with the infrared radiation. This procedure is not very sensitive, like all those based on the thermal variations of an optical parameter of a pure body.
By way of example of a reference illustrating the prior art French patent No. 71 08 811 (publication No. 2,081,937) may be cited, which discloses an image converter in which the liquid film, which is locally deformable as a function of the intensity of the infrared radiation, is supported by a freely deformable thin membrane.
The drawback common to all image converters utilizing a sensitive layer is that the latter must be permanently regenerated when the converter is being used. It is then necessary to provide, in addition, windshield wiper type devices which spread the layer on its support. The construction and use of such converters therefore necessarily pose technical problems. The support for the sensitive layer is not sufficiently strong to support the "windshield wiper" device. Moreover, the dimensions of the layer is perforce limited by imperatives of construction and renewal of the sensitive layer.
French patent No. 72 14 212 is concerned with a method comprising converting thermal images, particularly for viewing an object emitting thermal radiation, viz. infrared radiation, by forming a thermal image, namely that of the object, on a face of an interface which is covered with at least one layer of thermal radiation absorbing material, the distribution of the temperatures in the thermal image resulting in a variation of physical parameters of the layer.
The other face of the interface, which is reflective, is illuminated with a beam of parallel light rays the optical properties of which undergo during the reflection of the interface a corresponding variation of its optical properties. In the viewing plane a viewable secondary image exhibits the last named variation. The method according to the aforesaid French patent makes use of the index gradient and variation of the thickness of the layer of thermal radiation absorbing material.
To practice such a method the reflection of the beam of parallel light rays is effected on a plane solid-solid or solid-liquid interface. The interface is for example defined by the hypotenuse face of a prism on which is deposited a solid layer such as a methacrylic ester.